1) Field of the Invention
The present invention relates to an improvement in a frequency converter. This invention also relates to a radio communication apparatus that employs the frequency converter as an up-converter or a down-converter.
2) Description of the Related Art
Information devices, employing radio communications such as mobile phones and portable information devices, have recently attracted increasing research and development. Because of such research and development, the information devices are day-by-day becoming smaller and cheaper.
The information device becomes smaller or cheaper if analog radio frequency (RF) circuits in the information devices are made as integrated circuits. Frequency converters, such as single balanced mixers and double balanced mixers are the examples of such analog RF circuits. These type of mixers have been disclosed, for example, in “Monolithic RF Active Mixer Design”, K. L. Fong and R. G. Meyer, IEEE Transactions on circuits and systems-II: Analog and digital signal processing, vol. 46, No. 3, March 1999, pp. 231–239.
The mixers described in this document include a driver stage and a switching stage. If such mixers are used in a frequency converter, it is necessary to set a condition of conjugate power match in order to obtain the available maximum power for analog circuits such as a frequency converter. However, in the frequency converter that employs such mixers, such condition is not met since the driver stage and the switching stage have mainly capacitive load (−JX, where X is a positive number) in a high frequency.
As a result, the frequency converters that employ the mixers described in the document have poor conversion gain.